1. Field of the Invention
This invention relates to a multi-layer ceramic wiring circuit board. More particularly, it relates to a multi-layer ceramic wiring circuit board suitable for constituting a functional module by mounting pins or leads for input or output and semiconductor parts, and a process for producing such a circuit wiring board.
2. Description of the Prior Art
Ceramic substrates are increasingly used as insulating substrates for multi-layer wiring circuit boards for mounting semiconductor parts at high density. Most of these ceramic insulating substrates are made of a sintered body basically composed of alumina (Al.sub.2 O.sub.3), and a typical example of such substrate is shown in "A Multi-Layer Ceramic, Multi-Chip Module" in the proceedings of the 30th Electronic Component Conference of IEEE (1980), pp. 283-285. The most serious drawback of the substrates using an aluminabased sintered body as insulator is the low signal propagation speed. This is attributable to the following fact. Generally, signal propagation speed is inversely proportional to the square root of relative dielectric constant of the insulator surrounding the signal wiring. The relative dielectric constant of alumina is as high as about 10, so that the sintered body composed mainly of alumina necessarily has a high relative dielectric constant which is usually on the order of 9.0 to 10.0.
For eliminating such defect of low signal propagation speed, a substrate using an insulator with a small relative dielectric constant is needed. For instance, when a sintered body composed principally of mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) is used as insulator, a ceramic substrate with a relative dielectric constant of the order of 5.5 to 7.0 can be obtained. Also, if a sintered body having fine pores in the insulator becomes available, it will be possible to make the relative dielectric constant of the sintered body smaller than that intrinsic to the base material. However, such insulator substrate is low in strength and involves the problem of insufficient mechanical and thermal strength of the surface for mounting the pins or leads for input and output and for mounting and securing the electrical parts such as semiconductor parts.